Production of aromatic polycarboxylic acids



Patented Mar. 23, 1954 PRODUCTION OF AROMATIC POLYCAR- BOXYLIC ACIDS Martinus L. Goedkoop, Geleen, Netherlands, as-

signor to De Directie van de Staatsmijnen in Limburg, Heerlen, Netherlands No Drawing. Application March 21, 1951, Serial No. 216,840

Claims priority, application Netherlands March 28, 1950 10 Claims. (Cl. 260-515) This application relates to the manufacture of aromatic polycar'boxylic acids from carbonaceous material with a high carbon content, such as graphite, anthracite, coal, charcoal, coke, retort carbon, coal hydrogenation residue, pitch coke, coke from mineral oil products, pitch, and the like.

FIEID OF INVENTION Aromatic polycarboxylic acids, such as mellitic acid (benzene hexacarboxylic acid) and pyromellitic acid (benzene tetracarboxylic acid) can be obtained by oxidation of natural or artificial carbonaceous materials having a high carbon content with, for instance, nitric acid.

This type of oxidation is slow, but may be accelerated by adding catalysts, although, in any case, very long reaction periods are required. While attempts have been made to shorten the duration of the reaction by raising the temperature, this has produced difliculties in practice, because the temperature can only be raised if an elevated pressure is employed.

OBJECTS A principal object of this invention is the provision of new improvements in the general procedure for making aromatic polycarboxylic acids by oxidation of carbonaceous materials having high carbon content. Further objects include:

(1) The provision of improvements in this general type of procedure which greatly shorten the necessary reaction time.

' (2) The provision of procedures for making benzene polycar'boxylic acids from carbonaceous materials which may be carried out in relatively short periods of time and at atmospheric pressure.

(3) The provision of modifications in such known operations which allow for improved yield in desired product.

(4) The provision of such processes which make possible the recovery of valuble inorganic by-products, such as aluminum sulfate or the like.

(5) The use of a procedure in the oxidation of carbonaceous materials to polycarboxylic acids with nitric acid in such manner that the nitric acid values contained in the vapors which issue from the reaction mass may be recovered and recycled in the process.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however,

2 that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent tothose skilled in the art from this detailed description.

GENERAL DESCRIPTION These objects are accomplished in accordance with the present invention by a combination of steps which include:

(a) The formation of a suspension of subdivided, solid, carbonaceous material in sulfuric acid;

(b) The addition of nitric acid to this suspen sion;

(c) Heating of the resulting reaction mass to a temperature of between and 230 C.;

(d) Continued addition of nitric acid as required to replace that which is consumed by evolution from the reaction mass or by the reaction with the carbonaceous material at said temperature until the desired amount of carbonaceous material is oxidized, and

(e) Separation of the resulting solution of polycarboxylic acids from undissolved residue.

The undissolved residue separated from the acid solution, such as by filtration, may be treated, for example by washing, with water, in order to recover valuable inorganic by-products therefrom which, with most carbonaceous materials generally treated, will consist primarily of aluminum sulfate.

The success of the present invention is due, to a large extent, to the discovery that when the oxidation of the carbonaceous material is conducted in a medium of liquid sulfuric acid and at the temperatures indicated, a critical increase in the reaction rate of the process, as compared with previously used procedures, is obtained. Not only does reaction in this manner shorten the time required to produce the desired] polycarboxylic acids, but sible to recycle nitric acid, thus providing for a saving in valuable reagents.

EXAMPLES A more complete understanding of the new procedures, as broadly described above, may be had by reference to the following examples of actual operations in accordance with the invention.

Example I In a cylindrical reaction vessel (diameter 7 cm., height 50 cm.) nitric acid (specific gravity 1.4)

it has also: been found pow is continuously led at a rate of 700 ml. per hour into 500 ml. 96% sulfuric acid in which 350 grams of coke breeze are suspended. The temperature of the reaction mixture is maintained between 195 and 205 C., but after an initial heating, no further external heating is needed to maintain this temperature.

After a reaction time of 8 hours, the solution becomes a dark green color which, subsequently, changes into a light whitish-green color in which the ash components can be clearly distinguished. After the intial material has been converted into light-colored products the reaction is brought to an end. The introduction of the nitric acid is stopped and the hot reaction mixture filtered. On cooling the filtrate, the polycarboxylic acids, consisting chiefly of mellitic acid, crystallize out of the solution. A yield of 51 grams of lightyellow, crude mellitic acid is obtained.

The filtration residue is lixiviated with water and the resulting aqueous solution is dried by evaporation, giving 75 grams of inorganic byproduct, consisting chiefly of acid aluminum sulfate.

Example II The experiment described in Example I is re peated at an oxidation temperature of 170 C.

In this case, 81 grams of crude mellitic acid are obtained after a reaction period of 12 hours.

Example III action time of 8 hours, 102 grams of raw mellitic acid are obtained.

Example IV Using the reaction vessel of Example I, nitric acid of specific gravity 1.4 is added strearnwise to a suspension of 200 grams of petroleum coke in 600 ml. of 90% sulfuric acid. The temperature of the reaction is maintained at 215 C. After about 8 hours of streamwise addition of the nitric acid and maintenance of the temperature as indicated, the resulting solution of polycarboxylic acids is separated from insoluble residue and then the organic acid components of the solution are recovered by crystallization. A yield of 32 grams of raw mellitic acid is obtained.

Example V In the same manner as described in Example IV after a reaction time of 9 hours 120 grams of raw mellitic acid are obtained from 250 grams of pitch coke. The pitch coke used was obtained from a carbonisation at a temperature of 700-800 C. of pitch, which resulted from a high temperature carbonisation of coal at about 1000" C.

DETAILED DESCRIPTION The new procedures are applicable to all carbonaceous materials having a relatively high carbon content, either of a natural or artificial source. Examples of preferred materials have been given in the introduction hereof.

The maintenance of the temperature of reaction within the range of 130 to 230 C. during the process is important. At temperatures below this limit, the nitric acid remains in the liquid state and the increased reaction speeds are not obtained. The use of temperatures above this range have been found to reduce yields of desired product and to introduce other difliculties. A preferred temperature range is 170 to 215 C.

The nitric acid may be introduced either in the liquid or in gaseous phase. When nitric acid vapors are used, it is preferable to introduce these vapours into the suspension with the aid of a carrier gas, preferably an oxygen-containing carrier gas, such as air. The concentration of the nitric acidmay be varied. When dilute nitric acid is used, the oxidation proceeds more slowly than in the application of concentrated nitric acid. Nitric acid having a concentration of to gives excellent results, and With still higher concentrations up to even 100% very satisfactory results are obtained.

The concentration of the sulfuric acid may be varied also. It is not necessary to use strong sulfuric acid having a concentration of over and the procedure is still operative with dilute sulfuric acid of for instance 50% concentration.

The ratio of sulfuric acid to carbonaceous material in the suspensions treated by these new procedures can be varied, and the exact ratio used will depend to some extent upon the concentration of the sulfuric acid and the particularly carbonaceous material used. However, it has been found that best results are obtained if certain preferred ratios are employed. With sulfuric acid of usable concentration, the use of suspensions of between 10 and 50 parts by weight of carbonaceous material for each parts by weight of sulfuric acid is preferred. I

The ratio of nitric acid to carbonaceous material, and in turn, sulfuric acid, in the reaction mixture can be varied and will depend inpart upon the nitric acid concentration, the sulfuric acid concentration and theexact nature ofthe carbonaceous material under treatment. Under most conditions, however, the addition of nitric acid of any usable concentration at the rate of between 200 and 700 parts perhour for each 100 parts of carbonaceous material in the suspension gives the most desirable results.

The process is begun by the formation of a suspension of the carbonaceous material in the sulfuric acid. Generally, this suspension is formed before the nitric acid is added, but it is possible to begin the process by adding nitric acid to the sulfuric acid even before the carbonaceous material is suspended in the acid mixture. In either case, thereafter, nitric acid is added, generally continuously, e. g., streamwise, although, it may be added batchwise.

The reaction time depends on the nature of the starting material. When coke is used as a starting material, it takes a few hours before the organic components are oxidized and dissolved in the reaction mixture. If, however, coal is oxidized in this manner, a solution of polycarboxylic acids is obtained within a few minutes. In any event, the reaction is usually continued until the oxidation of the carbonaceous material is substantially completed, which can, as a rule, be determined by the change in color of the reaction mass. Best results are obtained with a reaction time between about 30 minutes and 12 hours. With coke, the most desirable reactiontime is about 8 to 12 hours.

Although it is possible to carry out the new procedures of this invention at superatmospheric pressures, this is unnecessary and, actually, undesirable, since the use of pressure apparatus retards the escape of nitric acid vapors from the liquid re-.-

action mixture and results in a lengthening of-the reaction time. i The greater part of the inorganic components of the-starting materialdoesnotdissolve in the reaction mixture; so that these components together with some unconverted starting material can bev separated from the reaction liquor in a simple manner. Apart from silicic acid,.the residue removed contains a valuable by product, consistingchiefiy of acid aluminum sulfate, ad.- mixed' With small amounts of sulfates of iron, potassiinn, magnesium and'calcium. treating it with water, th residue canbe' freed from the acid sulfates which can betseparated from the silicic acid for instance by filtration.

The amount of inorganic lav-product, depends on the. ash content of the initial material used. It dust .coal; with a low ash. content, consisting chiefly of fusain, is used as starting material, only a very small amount of by-product is formed.

It is preferred to remove the inorganic byproducts at about the reaction temperature because at lower temperatures, the polycarboxylic acids are less soluble in the reaction liquor. This may be effected for instance by filtration, whereafter on cooling the filtrate, the polycarboxylic acids consisting chiefly of mellitic acid are crystallized out.

The nitric acid vapors escaping from the reaction mixture can be re-used. By liquefying the vapors, the carbon dioxide present therein may be carried off, because in this operation, the carbon dioxide remains in the gaseous phase. In this manner, a cyclic process is obtained in which the nitric acid, which is continuously fed into the suspension, is recovered from the vapors which escape from the reaction mixture. By supplying concentrated nitric acid, the concentration can be kept constant.

Known expedients, not clearly antagonistic to the new procedures, used heretofore with this general type of operation, may be employed in the procedures of this invention. Thus, catalysts, designed to accelerate the reaction, may be incorporated in the reaction mixtures, although, the improved speeds of reaction made possible by this invention do not necessitate the use of such modifications. Examples of usable catalysts include, oxides of metals, such as molybdenum or vanadium. Other similar modification can also be utilized, if desired.

CONCLUSIONS The new improvements described herein for the manufacture of aromatic polycarboxylic acids, particularly, mellitic acid, from carbonaceous materials, such as coal, coke, petroleum coke, or the like, make it possible to carry out this generally known type of operation to form desired products in good yield in a much shorter period of time than has been possible heretofore. By these new operations, it is possible to recycle valuable reagents, such as nitric acid, and even possible to carry out the process in a continuous fashion. The recovery of resulting products from the reaction mixture is readily accomplished and, at the same time, where carbonaceous ma terial, containing inorganic components, is used, it is possible to recover valuable inorganic byproducts, such as aluminum sulfate.

I claim:

1. A process for the production of aromatic polycarboxylic acids by oxidation of carbonaceous materials which comprises forming a suspension of subdivided solid carbonaceous material selected from the group consisting of graphite, anthracite, coal, charcoal, petroleum coke, coke, pitch coke, retort carbon, pitch, and coal hydrogenation residue in sulfuric acid, adding nitric acid thereto,

6. heating the reaction mass to: a: tempera'tureibw tween 130 and 230 0;, continuing additiorroft nitric acid to said suspension until a substantial? amount of said carbonaceous materialis oxidized, and then separating the resulting solution of polycarboxylic acids from undissolved residue.

2. A process for the production of an aromatic mixture of polycarboxylic acidsconsisting chiefly of mellitic acid by oxidation of carbonaceous materials which comprises forming a suspension of finely subdivided solid carbonaceous material from the group consisting of graphite, anthracite, coal, charcoal, petroleum coke, coke, pitch coke, retort carbon, pitch, and coal hydrogenation residue in sulfuric acid having a concentration of at least 50%, adding nitric acid streamwise to the suspension, heating the reaction mass to a temperature between 130 and 230 C., continuing the addition of nitric acid to the reaction mass until substantially all of the carbonaceous material is oxidized, and then separating the resulting solution of acid mixture containing mellitic acid from the dissolved residue.

3. A process for the production of mellitic acid which comprises forming a suspension of coke in sulfuric acid having a concentration of at least 50%, adding streamwise to said suspension nitric acid having a concentration of about 65 to heating the mixture to a temperature of 130 to 230 C., continuing said addition of nitric acid for a period of about 8 to 12 hours, separating the resulting solution of mellitic acid from insoluble residue, and recovering mellitic acid from said separated solution by crystallization.

4. A process as claimed in claim 1, wherein the nitric acid evolved from the vapors escaping from the reaction mixture are recovered and said recovered nitric acid is recycled for addition to said suspension.

5. A process as claimed in claim 1, wherein the nitric acid in the vapor phase is added to the suspension.

6. A process as claimed in claim 5, wherein the nitric acid in the vapor phase is mixed with a carrier gas.

7. A process as claimed in claim 6, wherein said carrier gas is air.

8. A process for the production of mellitic acid which comprises forming a suspension of carbonaceous material selected from the group consisting of graphite, anthracite, coal, charcoal, petroleum coke, coke, pitch coke, retort carbon, pitch, and coal hydrogenation residue in sulfuric acid of a concentration of at least 50 adding nitric acid of a concentration of about 65 to streamwise to said suspension, heating the reaction mass to a temperature between and 230 C., continuing the addition of nitric acid while the mass is maintained at said temperature until substantially all of the carbonaceous material has been oxidized, separating the resulting solution of mellitic acid from undissolved inorganic residue while the reaction mass is hot enough to prevent crystallization of the mellitic acid and then separating crude mellitic acid from the recovered solution by crystallization.

9. A process for the production of mellitic acid which comprises forming a suspension of finely subdivided coke in sulfuric acid having a concentration of 90 to 96%, adding nitric acid of concentration of 65 to 70% in the vapor phase admixed with air streamwise to said coke suspension, heating the reaction mass to a temperature between and 215 C., continuing the addition of the nitric acid to the suspension maintained at said temperature for about 8 to 12 hours, 'sep 'ating'the hot resulting supernatant liquor from e insoluble residue, and recovering mellitic acid mm the filtrate by crystallization.

10. A process as claimed in claim 1, wherein said oxidation is effected at atmospheric pressure.

MARTINUS L. GOEDKOOP.

References Cited in the file of this patent UNITED STATES PATENTS Number Number Name Date Gontard et a1 Oct. 19, 1920 Juettner Oct. 17, 1939 Leicester Mar. 12, 1940 Kiebler Feb. 15, 1949 Howard June 5, 1951 FOREIGN PATENTS Country Date Great Britain 1907 Germany Mar. 4, 1927 France Feb. 16, 1943 

1. A PROCESS FOR THE PRODUCTION OF AROMATIC POLYCARBOXYLIC ACIDS BY OXIDATION OF CARBONACEOUS MATERIALS WHICH COMPRISES FORMING A SUSPENSION OF SUBDIVIDED SOLID CARBONACEOUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF GRAPHITE, ANTHRACITE, COAL, CHARCOAL, PETROLEUM COKE, COKE, PITCH COKE, RETORT CARBON, PITCH, AND COAL HYDROGENATION RESIDUE IN SULFURIC ACID, ADDING NITRIC ACID THERETO, HEATING THE REACTION MASS TO A TEMPERATURE BETWEEN 130* AND 230* C., CONTINUING ADDITION OF NITRIC ACID TO SAID SUSPENSION UNITL A SUBSTANTIAL AMOUNT OF SAID CARBONACEOUS MATERIAL IS OXIDIZED, AND THEN SEPARATING THE RESULTING SOLUTION OF POLYCARBOXYLIC ACIDS FROM UNDISSOLVED RESIDUE. 